Method and installation for treating plastic tubes with bi-axial drawing

ABSTRACT

The invention concerns a method for treating plastic tubes, in particular PCV tubes, obtained by extrusion, whereby a tube is brought to a molecular orientation temperature, higher than room temperature, and is subjected to bi-axial drawing by radial expansion and axial elongation, to obtain a bi-oriented tube whose mechanical properties are improved. The tube (T) is cut into elements ( 4 ) of predetermined length immediately after being extruded and its temperature is higher than the molecular orientation temperature, each element constituting a blank ( 5 ); each blank ( 5 ) is placed in a chamber (E) to be cooled therein by a fluid (F) to a temperature close to the molecular orientation temperature, the dwell time of the blank in the chamber being greater than the time of its production by extrusion; and the blank ( 5 ) is then extracted from the chamber (E) to be subjected to a treatment enabling its bi-orientation.

The invention relates to a method for the processing of plastic tubesobtained by extrusion, the said method being of the type according towhich a plastic tube is brought to a molecular orientation temperaturehigher than the ambient temperature and is subjected to biaxialstretching by radial expansion and by axial elongation, in order toobtain a bi-orientation tube which has improved mechanical properties.

Such a method is known, particularly from the documents GB-A-1,432,539or U.S. Pat. No. 4,340,344. The additional processing of the plastictubes which this method necessitates entails an appreciable increase inthe prices of the finished products. However, the great improvement inthe mechanical characteristics of the products, in particular increasein mechanical resistance to internal pressure, is conducive to employingthis method for products, in particular tubes intended for conveyingfluids under pressure, which must have good mechanical resistance.

According to the known method, the tubes manufactured by extrusion arestored at ambient temperature, are subsequently taken from stock and arebrought by heating from the ambient temperature to the molecularorientation temperature in order to undergo the processing. The heatingof a tube to be processed is relatively lengthy and involves a highenergy consumption, since it is necessary to cause the tube to changefrom the ambient temperature to the molecular orientation temperature.The result of this is that the efficiency of the processing method isnot very high and needs to be improved.

The object of the invention is, above all, to provide a method for theprocessing of tubes made of plastic, especially PVC (polyvinylchloride), as defined above, which allows higher efficiency, inparticular by a reduction in the processing time and by a decrease inthe energy consumption necessary for processing.

It is desirable that the method should make it possible to improve thetemperature profile in the wall of the tube for the purpose of thebiaxial stretching of the latter.

It is desirable, furthermore, that the processing method should remainrelatively simple and not require too great an amount of floor space, sothat the cost of investments for carrying out the method remainsacceptable.

According to the invention, a method for the processing of tubes made ofplastic, especially PVC, obtained by extrusion, according to which aplastic tube is brought to a molecular orientation temperature higherthan the ambient temperature and is subjected to biaxial stretching byradial expansion and by axial elongation, in order to obtain abi-orientation tube having improved mechanical properties, ischaracterized in that:

the tube is cut into elements of specific length when it has just beenextruded and when its temperature is higher than the molecularorientation temperature, each element constituting a blank;

each blank is placed in an enclosure in order to be cooled there by afluid at a temperature near the molecular orientation temperature, thedwell time of the blank in the enclosure being longer than the time forproducing the blank by extrusion, in order to allow a correcttemperature setting.

Should this dwell time be considered too long, it is possible for theblank to be cooled sharply, but briefly, before it is introduced intothe enclosure, by spraying or by means of a bath of cold water. A thinlayer of material could then be temporarily at a temperature below thetemperature desired for the bi-orientation operation.

The blank is subsequently extracted from the enclosure in order toundergo the processing which ensures bi-orientation.

Preferably, just after being extruded and before being cut, the tube iscooled superficially by passing through a cooling fixture, in order toensure a clean cut of the still hot tube.

The tube is advantageously cut by means of a shearing operation with ablade device of the guillotine type.

The fluid enclosure is advantageously formed by a tank of hot waterwhich, in the case of PVC processing, is at a temperature near or equalto its boiling temperature at atmospheric pressure.

It is desirable to ensure the best (and uniform) transfer of heatbetween the fluid and an article of complex shape (the exterior andinterior of a tube of great length). For this purpose, preferably, theblanks are agitated in the enclosure. Advantageously, the fluid iscirculated in the enclosure, in particular in a direction parallel tothe axis of the tube.

The invention also relates to an installation for carrying out themethod for the processing of plastic tubes, as defined above.

An installation for the processing of tubes made of plastic, especiallyPVC, according to the invention comprises an extruder for producing atube and means for subjecting the tube to biaxial stretching by radialexpansion and by axial elongation, in order to obtain a bi-orientationtube having improved mechanical properties, and is characterized in thatit comprises:

cutting means at the exit of the extruder, for delivering the tube inelements of specific length when it has just been extruded and when itstemperature is higher than the molecular orientation temperature, eachcut element constituting a blank;

an enclosure for receiving the blanks and cooling them by means of afluid at a temperature near the molecular orientation temperature, theenclosure being provided for ensuring a dwell time of the blank longerthan the time for producing the blank by extrusion, in order to allowthe correct temperature setting,

and means for extracting the blank from the enclosure in order to causeit to pass to the means for carrying out biaxial stretching.

A cooling fixture may be provided at the exit of the extruder, in orderto cool the tube superficially and allow a clean cut of the still hottube.

The cutting means advantageously consist of a cutting-blade device ofthe guillotine type, in particular fastened to the cooling fixture.

The fluid enclosure may be formed by a tank of hot water which, in thecase of PVC processing, is at a temperature near or equal to its boilingtemperature at atmospheric pressure.

Means for circulating the fluid in the tank, in particular in the axialdirection of the blanks, are provided.

Preferably, the blanks are agitated in the enclosure.

Means for setting in rotation the blank which will be extracted nextfrom the tank may be provided.

Advantageously, a roller conveyor or a belt device is arranged at theexit of the extruder, in alignment with the latter, in order to receivethe tube. The speed of the conveyor is substantially equal to the exitspeed of the tube from the extruder, in order to reduce the stresses onthe emerging tube to a minimum. The speed of the conveyor may bevariable around this condition for the purpose of overthickening (orunder-thickening) the tube blank locally.

The fluid enclosure for setting the temperature of the blank ispreferably placed on one side of the conveyor at a lower level. Pushingmeans are provided for displacing the blank transversely and causing itto enter the fluid enclosure.

Apart from the arrangements described above, the invention consists ofsome other arrangements which will be discussed more explicitly belowwith regard to an exemplary embodiment which is described with referenceto the accompanying drawings, but which is in no way limiting.

FIG. 1 of these drawings is a simplified diagrammatic top view of aninstallation for the manufacture of plastic tubes which carries out theprocessing method of the invention.

FIG. 2 is a simplified diagrammatic elevation view of the installationof FIG. 1.

FIG. 3 is a cross-section through an enclosure for the correcttemperature setting of the blanks, this enclosure being located next toa conveyor.

FIG. 4 is an elevation view of the device for cutting the tubes afterextrusion.

Finally, FIG. 5 is a view from the left in relation to FIG. 4.

Referring to the drawings, in particular to FIGS. 1 and 2, aninstallation 1 carrying out the method of the invention may be seen.

The installation comprises an extruder 2, shown diagrammatically, whichis supplied with thermoplastic granules, especially with PVC granules,from a hopper H shown diagrammatically. As is well known, the twineffects of heat and pressure within the extruder make it possible toagglomerate the plastic granules and obtain, at the exit of the land 3of the extruder, a tube blank or tube T, the wall of which is relativelysoft and at a relatively high temperature. With regard to PVC, thetemperature of the wall of the tube at the exit of the extruder is ofthe order of 180° C. to 200° C.

In a conventional installation (not shown), the extruder is followed bya unit (not shown) ensuring progressive cooling of the tube T whichbecomes rigid in proportion to this cooling. At the exit of such acooling unit, the tube, which has not undergone the processing ensuringthat it has bi-orientation, is delivered in portions of specific lengthand stored at ambient temperature. The portions thus prepared aresubsequently picked up again in order to undergo the bi-orientationprocessing after prior heating to the molecular orientation temperature.The cutting of the tube at the exit of the cooling unit takes placerelatively easily because the tube is rigid.

The length of the cooling unit depends on a plurality of parameters, inparticular on the thickness of the tube emerging from the extruder. As anon-limiting indication, the thickness range of tube blanks T may befrom a few millimetres up to 40 or 50 mm. In the case of tubes having athick wall, for example of the order of 20 mm, at the exit of theextruder 2, the length of the cooling unit would become very great andwould entail high expenditure.

According to the invention, to overcome these disadvantages, the tube Tis cut into elements 4 of specific length when it has just been extrudedand when its temperature is higher than the molecular orientationtemperature, each element thus cut constituting a blank 5.

Experience has shown that such a cut can be made correctly, inparticular, by means of shears 6 of the guillotine type which will bedescribed in more detail with reference to FIGS. 4 and 5.

Moreover, a cooling fixture 7 of small length, in particular less than0.50 m, is advantageously provided at the exit of the extruder andensures superficial cooling of the tube so as to set the outer layer ofthe tube and ensure a clean guillotine cut.

The tube emerging from the extruder 2 and from the cooling fixture 7 isreceived on a roller conveyor 8 arranged in the axial extension of theextruder 2. The speed of the conveyor 8 is substantially equal to theexit speed of the tube T from the extruder, in order to reduce to aminimum the tensile or compressive stresses on the emerging tube. Itshould be noted that it is possible to overthicken the tube blank Tlocally by slowing the conveyor 8 at the moment when the materialconstituting the desired zone passes into the cooling fixture 7.Conversely, the tube blank may be under-thickened locally byaccelerating the conveyor 8 at the appropriate moment.

The superficial cooling of the tube, initiated in the cooling fixture 7,may continue above the conveyor 8 by means of a sprinkler bank (notshown) which spreads water over the blank 5. Where PVC is concerned, thesurface temperature may then be in the neighbourhood of 70° C.

Each blank 5 is subsequently placed in an enclosure E arranged on oneside of the conveyor 8, in order to be cooled there by a fluid F whichis substantially at the molecular orientation temperature of the plasticof the blank. In the case of tubes made of PVC, the molecularorientation temperature of which is within a range of approximately 90°C. to 110° C., an advantageous solution involves using the hot water asa cooling fluid at a temperature near its boiling temperature atatmospheric pressure, namely 100° C. In an exemplary embodiment, thewater of the tank was at a temperature of between 95° C. and 98° C.

The water from the tank, although being near its boiling temperature,acts as a cooling fluid, since the blank emerging from the extruder isat a temperature of approximately 180° C., in the case of PVC, and sinceonly the superficial zone has been cooled. Thermal self-regulationoccurs in the tank 9 by evaporation of the water.

The dwell time of the blank 5 in the tank 9 is sufficient to allowcorrect temperature setting, that is to say homogenization of thetemperature of the blank, essentially in the longitudinal direction andthe peripheral direction. The temperature in the thickness of the tube,or radial direction, will be referred to later. The dwell time is longerthan the time necessary for producing the blank by extrusion; the dwelltime may be of the order of 20 to 30 minutes for PVC tubes, of which thethickness at the exit of the extruder is of the order of 20 mm. The timefor producing a blank by means of the extruder is of the order of a fewminutes, for example 3 to 5 minutes. Consequently, the tank 9 isdesigned for storing a sufficient number of blanks and is adapted to theextrusion processing time and to the time for subsequent bi-orientationprocessing.

Temperature setting thus takes place with a tank 9 of reduced length.

Advantageously, the hot water is circulated within the tank by means ofpumps 10, 11 and pipelines 12 connecting longitudinal end zones of thetank to the intake and to the delivery of each pump. The assembly as awhole is arranged so that the circulation of water in the tank 9 takesplace in a general direction parallel to the length of the tank and tothe axis of the blanks 5 immersed in this tank. The pump 11, located onthe right-hand side of the tank 9 according to the drawings, isequipped, at its delivery, with an injector 13 positioned so as to sendthe water jet into the blank 5 a which is the oldest in the tank 9 andwhich will be the next to be extracted from the tank. According to FIG.3, this blank 5 a is located at the left-hand end of the juxta-positionof blanks 5. Moreover, in order to improve further the homogenization ofthe temperature of the blank 5 a, means 14 for driving this blank 5 a inrotation are provided. These means 14 may consist of two rollers whichare parallel to the axis of the blank and one of which is a drivingroller.

The means for driving in rotation 14 are supported by arms 15 in theform of an upturned swan neck. Such arms 15 are distributed insufficient number in the longitudinal direction of the blank 5 a and arefastened to a support 16 which can be raised relative to the frame ofthe tank 9 by means of one or more jacks 17, in such a way as to extractthe blank 5 a from the tank 9.

It should be noted that the temperature of the blank 5 a is homogenizedessentially in the longitudinal direction and the peripheral direction.In the radial direction (thickness) of the blank, it is not necessaryfor the temperature to be homogeneous. Since the immersion time isrelatively short (20 to 30 minutes), the temperature of the PVC at thecentre (mid-way in the thickness of the blank) is at an intermediatevalue between the extrusion temperature (approximately 200° C.) and thetemperature of the water (approximately 100° C.). This contributes toimproving the subsequent biaxial stretching processing.

Furthermore, means P, merely indicated in FIGS. 1 and 2, are providedfor displacing the blank 5 a, raised from the tank, in the longitudinaldirection and for introducing it into a unit 18, where it will undergoradial expansion and axial elongation leading to bi-orientation and toan improvement in the mechanical properties of the material. A hoist 19is provided between the hot-water tank 9 and the unit 18. The hoist 19is equipped with a guide 20 which, in the high position, supports theblank during its transfer from the arms 15 to the unit 18.

Preferably, the blanks 5 stored in the tank 9, with the exception of theblank 5 a next to be extracted, are supported by transverse arms 21mounted pivotably about an axis 22 parallel to the longitudinaldirection of the blanks. The arms 21 are actuated in an alternatingoscillating movement about the axis 22 by means of a drive system 23comprising a motor 24 driving an eccentric 25 which is connected to oneend of the arm 21 by means of a connecting rod 26.

As may be seen in FIG. 3, the conveyor 8, which receives the tube at theexit of the extruder 2, is placed parallel to a large side of the tank 9in the upper region of this tank. A transverse pushing device 27 isprovided for transferring the blank 5 into the tank 9 where it falls asa result of gravity.

A tilting flap V (FIG. 3) is provided for allowing the blank 5 to pass,whilst preventing it from falling into the space between the tank 9 andconveyor 8.

The guillotine shears 6 are illustrated in more detail in FIGS. 4 and 5.These shears comprise a blade 28, the cutting edge of which is obliqueand which can be displaced vertically by means of a jack 29. In contrastto the diagrammatic illustration in FIGS. 1 and 2, this cutting blade 26is located at the exit of the cooling fixture 7, of which the enddistant from the guillotine 6 is equipped with a flange 30 for fasteningto the exit of the extruder. The cooling fixture 7 comprises acylindrical double casing defining an annular chamber 31, in which thecooling water circulates, the plastic tube passing into the innercylindrical casing 32 of the cooling fixture 7.

This being so, the installation functions as follows.

The extruder 2, fed with raw material granules or powder by the hopperH, supplies at the exit a soft-walled tube T which, by passing throughthe cooling fixture 7, is cooled on its surface. The tube T is receivedon the conveyor 8. When the tube length reaches a predetermined value(for example, 6 meters), the guillotine cutting device 6 cuts off anelement 4 which continues its travel on the conveyor 8 until it is inposition to be ejected into the tank 9.

The pushing device 27 is then triggered in order to push the element4/blank 5 towards the tank 9 transversely relative to the conveyor 8.

The last blank 5 which has entered the tank 9 is located on the rightaccording to FIG. 3. This blank will dwell for a time equal to amultiple of the time necessary for the extrusion of an element and willbe displaced progressively in the transverse direction towards therollers 14. When the blank arrives at 5 a on the rollers 14, it is setin rotation about its axis and a stream of water coming from theinjector 13 passes through it along its axis.

The blank 5 a is subsequently raised by the arms 15 and is pushedlongitudinally by the device P in order to pass onto or into the guide20 and enter the processing unit 18 which is in alignment with the blank5 a raised by the arms 15.

The method and installation of the invention allow in-line processing ofthe tubes, that is to say the bi-orientation processing can be carriedout consecutively to the extruder. The intermediate tank 9, by storingseveral blanks, makes it possible to set the temperature correctly,whilst ensuring a reduced overall size.

The energy saving is appreciable, since the blank, which is at atemperature higher than the molecular orientation temperature, isbrought to the latter temperature directly, without being cooled toambient temperature, which would have necessitated subsequent reheating.

The extrusion installation is simplified, since it does not have eithera unit for ensuring substantial cooling or a device for drawing thetube.

The invention has been described by reference to the example of PVC, butit is clear that it applies to other plastics capable of undergoingsimilar processing, for example polyethylene or polypropylene. It isexpedient to regulate the temperature of the fluid of the enclosure Eaccording to the plastic.

The fluid used in the enclosure E may, of course, be different fromwater. It is possible, for example, to complete or replace the action ofthe water by air or by oil.

What is claimed is:
 1. Method for the processing of tubes made ofplastic, obtained by extrusion, according to which a tube is brought toa molecular orientation temperature higher than ambient temperature andis subjected to biaxial stretching by radial expansion and by axialelongation, in order to obtain a bi-orientation tube having improvedmechanical properties, characterized in that: the tube (T) is cut intoelements (4) of specific length when it has just been extruded and whenits temperature is higher than the molecular orientation temperature,each element constituting a blank (5); each blank (5) is placed in anenclosure (E) in order to be cooled there by a fluid (F) at atemperature near the molecular orientation temperature, the dwell timeof the blank in the enclosure being longer than the time for producingthe blank by extrusion, in order to allow correct temperature setting,and the blank (5 a) is subsequently extracted from the enclosure (E) inorder to undergo the processing which ensures bi-orientation.
 2. Methodaccording to claim 1, characterized in that the tube (T), just afterbeing extruded and before being cut, is cooled superficially by passingthrough a cooling fixture (7), the length of which is less than 0.50 m.3. Method according to claim 1, characterized in that the tube is cut bymeans of a shearing operation with a guillotine device (6) with a blade(28).
 4. Method according to claim 1, characterized in that the fluidenclosure (E) is formed by a hot-water tank (9).
 5. Method according toclaim 4, characterized in that the temperature of the water is near 100°C.
 6. Method according to claim 5, characterized in that the watertemperature is within a range of 95° C. to 98° C.
 7. Method according toclaim 4, characterized in that the blanks (5) are agitated in theenclosure (E).
 8. Method according to claim 1, characterized in that thefluid is circulated in the enclosure, in particular in a directionparallel to the axis of the tube.
 9. Method according to claim 1,characterized in that the plastic is PVC.